Hesham elhalawani, neil chevli, pamela K. Allen * & caroline chung * treatment for glioblastoma (GBM) includes surgical resection and adjuvant radiotherapy (Rt) and chemotherapy. the optimal time interval between surgery and Rt remains unclear. the national cancer Database (ncDB) was queried for patients with GBM. overall survival (oS) was estimated using Kaplan-Meier and log-rank tests. Univariate (UVA) and multivariable cox regression (MVA) modeling was used to determine predictors of OS. A total of 45,942 patients were included. On MVA: younger age, female gender, black ethnicity, higher KpS, obtaining a gross total resection (GtR), MGMt promotermethylated gene status, unifocal disease, higher RT dose, and RT delay of 4-8 weeks had improved oS. patients who underwent a subtotal resection (StR) had worsened survival with Rt delay ≤4 weeks and patients with GtR had worsened survival when Rt was delayed >8 weeks. This analysis suggests that an interval of 4-8 weeks between resection and RT results in better survival. Delays >8 weeks in patients with a GtR and delays <4 weeks in patients with a STR/biopsy resulted in worse survival. This impact of time delay from surgery to Rt, in conjunction with extent of resection, should be considered in the clinical management of patients and future designs of clinical trials. Glioblastoma (GBM) is the most common primary malignant brain tumor in adults. Current standard of care treatment for patients 70 years or younger and with good performance status, per the National Comprehensive Cancer Network (NCCN), includes maximal safe surgical resection with image-verified complete resection, followed by adjuvant radiotherapy (RT) and chemotherapy 1. Several prior studies have demonstrated an improved progression-free survival with more complete surgical resections 2-6. Under the same assumption that maximum cytoreductive treatment provides benefit to patients and, given the aggressive and rapidly progressive nature of this disease, many clinicians seek to minimize the time delay between surgery and initiation of RT. However, in the modern era, there are potentially numerous factors that could delay time to initiation of RT, including: evolution of practice that incorporates molecular/genetic testing and consideration of enrollment to clinical trials that requires additional testing and even central pathology review. This study provides information about the possible clinical impact of time delays for these various reasons between surgical resection and the start of adjuvant radiation therapy for patients with GBM. The optimal time interval between surgery and the initiation of adjuvant therapy; however, remains unclear. Cancer in several non-central nervous system (CNS) sites, including head and neck cancer and breast cancer, have increased local-regional recurrence rates when adjuvant RT is delayed 7. To our knowledge, in patients with GBM, two retrospective series have indicated that a delay in initiating adjuvant therapy worsened survival 8,9 ; nine series, including a SEER ana...